Battery-driven watch with battery consumption display alarm

ABSTRACT

The invention provides an advance alarm means incorporated with an electronic watch driven by a battery and fitted with a digital time display means. There is provided a detector sensing an abrupt voltage drop at the source battery. A selector represents its two states corresponding to the normal operating condition of the battery and to the voltage dropped condition thereof, respectively. This selector controls the feed of two different signal series to at least one electrically illuminatable elements of the digital time display means, depending upon the state of the selector and for providing an advance alarm before occurrence of an unacceptable voltage drop in the source battery.

United States Patent [191 Takamune et a1.

[54] BATTERY-DRIVEN WATCH WITH BATTERY CONSUMPTION DISPLAY ALARM [75]inventors: Hirotoki Takamune; Fuiio Ishida,

both of Tokyo, Japan [73] Assignee: Citizen Watch Company, Limited,

Tokyo, Japan [22] Filed: Nov. 8, 1973 [21] Appl. No.: 413,951

[30] Foreign Application Priority Data Nov. 9, 1972 Japan 47-112465 Nov.20. l972 Japan 47-1 16312 [52] US. Cl. 58/23 BA; 58/50 R; 58/152 H;340/248 B [51] Int. Cl. ..G04C 3/00; G04B 19/30; G08B 21/00 [58] FieldofSearch...58/23 R, 23 BA, 50 R, 152 H, 58/249; 340/248 B [451 Aug. 12,1975 Primary Examiner-Edith Simmons Jackmon Attorney, Agent, orFirmHolman & Stern 5 7] ABSTRACT The invention provides an advance alarmmeans incorporated with an electronic watch driven by a battery andfitted with a digital time display means.

[56] "R f c: d

UNITE; $2,32 LIZTENTS 6 Claims, 9 Drawing Figures 3,672,l55 6/1972Bergey et a1. ..58/5()R 62 I04 osc. E COUNTER DECORDERL IE 51 I5 L 63 a465 PULSE MIXER 69 u O2 l 3 SELECTOR y I WATCH DRIVE 23 7 CK 3| 5 2|AND'EATE DISPLAY TE ELEMENT i 22 34 v00 J 'i- OLTAGE SOURCE PATENTEDIIIIII 21975 3 8 7 9 O SHEET 1 FIG. l A

Z IoI LIJ 2 2% TIME (I) 661 68 I PULSE GEAR 5 MOTOR TRAIN a )62 n n u Ifn F FREQUENCY i osc. DMDER I COUNTER DECORDER IE /I;I I? 6k! L L63 L64 565 PULSE MIXER 69 |O2- -IO3 m VDD I SELECTOR WATCH URN? l 3| CKT. 33 l 4/5 2| DISPLAY ELEMENT 22 i i I VDD l 34 .J i" /I,VOLTAGE L2 M I SOURCEDETECTOR I J FIG. 3 /6A IO4A VOLTAGE: I SOURCE BATTERY-DRIVEN WATCH WITHBATTERY CONSUMPTION DISPLAY ALARM BACKGROUND OF THE INVENTION Thisinvention relates to a battery-driven watch fitted with a built-in alarmmechanism'for alarming beforehand the consumed condition of the battery.

A predominant drawback of the battery-driven watch is such that thebattery could be consumed up without alarming beforehand theconsuming-up condition of the battery to the watch-carrying person.Conventionally, the watch owner will become aware of the consumedbattery only upon it has been consumed up and the watch fallen into itsinoperating state.

If there is substantially no battery change facility in such a case thatthe watch owner should make a trip to far from the town, as an example,he can not utilize the time-indicating service of the watch.

SUMMARY OF THE INVENTION way of several preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a representative characteristic curve of a battery wherein thevoltage is plotted against the battery operating time.

FIGS. 2 2a and 3 5 are connection diagrams of first to fourthembodiments of the invention, respectively.

FIG. 6 is a chart showing several wave forms appearing at several partsof a fifth embodiment shown in FIG. 8.

FIG. 7 is a chart plotted in the similar manner to FIG. 1, in a betterform adapted for the illustration of operation of the fifth embodiment.

FIG. 8 is a similar view to FIG. 2, illustrating the fifth embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the accompanyingdrawings, several preferred embodiments of the invention will bedescribed in detail hereinbelow.

In FIG. 1, a typical characteristic curve of a mercuryor silver batteryis shown wherein the voltage is plotted level over a rather longoperating period until a sharp and abrupt voltage drop as at willappear, as was referred to hereinabove. At the arrival of this voltagedrop point 100, the dry battery must be discarded as consumed up.

In FIG. 2, a voltage drop detector 2 and a selector 3 are shownschematically as coupled to a regular electronic watch movement drivecircuit 6. In this embodiment, 1 represents a drive battery adapted forenergization of all the circuit components to be described. The detectorcircuit 2 comprises a fixed resistor 23, a variable resistor 22 and aninverter 21 electrically connected one after another as shown; saidinverter 21 may preferably be a MOS-semiconductors.

The selector circuit 3 comprises AND-gates 31 and 32, a NOR-gate 33 andan inverter 34; these circuit elements are electrically connected witheach other and with the circuits 2 and 6 as shown. The output of NOR-gate 33 is electrically connected through an amplifier 4 to a displayelement 5 which may be of the liquid crystal, illuminating diode or thelike type.

The AND-gates 31 and 32 are controlled by output signals from thedetector 2 for allowing selective passage of the pulses delivered fromthe drive circuit 6 and for periodical on-off operation of the displayelement 5, as will be more fully described hereinbelow.

The watch movement drive circuit 6 of the conventional type comprises anoscillator 61 and a frequency divider 62 which combination is designedand arranged, so as to deliver an electric voltage pulse per second, asan example. When the timepiece is of the digital type, the circuit 6further comprises a counter 63, a decoder 64 and a digital displaydevice 65 which are connected as shown. Or alternatively and as shown inFIG. 2a, when the watch is of the analog type, the circuit 6 comprises apulse motor 66, a regular timeindicating gear train 67 and a timedisplay mechanism 68, in place of and as direct substitutes for theaforementioned digital constituents 63 65, although these elements 63 68have been shown only schematically by virtue of their very popularityamong those skilled in the art.

The drive circuit 6 comprises further a pulse mixer 69, onlyschematically shown, adapted for delivery of seconds pulse series andhalf second pulse series having properly selected pulse width, in thepreferred embodiment so far shown and described. The former pulse seriesis delivered through first output lead 102 and the latter pulse seriesis delivered through second output lead 103 which extend from therespective outputs of the mixer 69 to the inlets of AND-gates 32 and 31,respectively.

The operation of the first embodiment is as follows:

By manipulating the variable resistor 22, the input voltage to inverter21 is adjusted to such level as slightly higher than the thresholdvoltage of an input transistor, not shown, contained in the inverter,upon applying the regular source voltage from the drive battery ltoinput terminal VDD of the detector 2. The resultant resistance of theseries-connected resistors 22 and 23 is selected to be a higher level aspossible from the view point of power consumption efficiency, so far asallowable without giving rise to instability of the detector operation.

By the establishment of the aforementioned operational conditions, theoutput from first inverter 21 will become binary 0, so far as the drivebattery 1 delivers battery capacity:

its regular voltage, while the output from second inverter 34 willbecome binary I, thus, the AND-gate 32 being kept in its conductingstate. Therefore, the display element 5 will periodically ignite withsecondsseries pulses fed from the drive circuit 6 through first lead102.

On the other hand, if the source voltage should drop abruptly as at 100in FIG. 1, the output from first inverter 21 will become binary l andfirst AND-gate 31 is brought into its conducting state. Thus, thedisplay element 5 will periodically ignite at a quicker frequency withthe half-second pulse series conveyed from the drive circuit 6 throughsecond lead 103. In this respect, it should be noted that the element 5is shown in triplicate in FIG. 2.

It will be seen from the foregoing that the display element 5 performs aperiodic flashing operation with seconds pulse series when the batterykeeps its normal operating voltage and with half second pulse serieswhen and after the battery voltage has dropped abruptly from the normaloperating voltage.

By slight modification of the circuit arrangement although not shown onaccount of easy occurrence to any person skilled in the art, the halfsecond pulse series to be delivered in the latter case may be modifiedto either slower or quicker mode than that above specified, so far as itcan be discriminated from the regular seconds pulse series. As anexample, the alarm signal pulse series may have a period of severalseconds or of a second divided by an integer larger than 2, as the casemay be.

On the other hand, the regular seconds pulse series may be modified soas to have a different period than a second and as occasion may desire.

From the data raised in this Table I, it will be seen that if thedetector has been designed to detect a 50 mV-voltage drop and theelectronic watch drive circuit can operate with l.4 volts at the lowest,an alarm period longer than a week may be provided until the watchmovement ceases its operation.

A more specific example of several consumption characteristic data of anelectronic watch drive battery is shownin the following Table 1:

Table l 150 mA h; longer than 360 days, 1.56 volts;

days between voltages I56 and 1.50; 7 days between voltages 1.50

and 1.40; 3 days between voltages 1.40

and 1.30;

normal effective life: normal working voltage: remaining working period:

Next, referring to FIG. 3, the second embodiment of the invention willbe described in detail.

In this Figure, numeral 1A represents again the drive battery having theoutput terminal VDD acting as an input to the detector circuit 2 whichcomprises fixed resistor 23, variable resistor 22 and inverter 2]arranged in the similar manner as before.

Numeral 6A represents the electronic watch movement drive circuitdesigned and arranged as at 6 in the foregoing first embodiment, yetbeing devoid of pulse mixer 69. The output of the drive circuit 6A iselectrically connected through lead means 104A to conventional digitaldisplay device 65A which is similar in its design and function as thoseof the device which was shown at 65 in the foregoing. The device 65 or65A comprises a number of electro-optical display elements which areenergizable by impressed voltages as commonly known supplied from theoutputs of the drive circuit 6 or 6A and now shows a specific time pointof 12 hours 15 minutes, as an example.

The output of inverter 21 is connected through an amplifier 41 to anilluminatable alarm 5A which is provided on the display device 5A.

The operation of the second embodiment is as follows:

When the battery 1 keeps its normal operating voltage, the output frominverter 21 will become binary 0 and thus, the illuminatable alarm 5Adoes not ignite.

On the other hand, when the battery voltage shows an abrupt drop as atin FIG. 1, the output from inverter 21 will represent binary l and thusthe alarm 5A will ignite for providing a visual alarm signal to thebearer of the watch.

In FIG. 4, the third embodiment of the invention is illustrated.

In this Figure, numerals l, 2, 21, 22 and 23 represent respectivesimilar constituents as before. This is applied to FIG. 5 showing thefourth embodiment. In the similar way, numerals 6B, 61 B, 62B, 63B, 64Band 65B represent respective similar constituents denoted 6, 61, 62, 63,64 and 65 as before.

The conventional digital display device comprises illuminatable timedisplay elements 518 which are energized by output voltages from thewatch movement drive circuit 68.

Numeral 38 represents a selector circuit which comprises a NAND-gate 35having two inputs. One of these inputs is connected with a junctionpoint positioned between two blocks 62B and 62C, while the other inputis connected with the output of inverter 21.

7 represents a switching transistor which is so connected although notspecifically shown as to earth the common negative terminal to theilluminatable time display elements 518, or to interrupt the earthconnection, depending upon the implied input from NAND- gate 35.

With the said arrangement, the output from inverter 21 will becomebinary O and the output from NAND- gate 35 will become binary I when theworking voltage of battery 1 is at the normal level so that theilluminatable display elements 518 perform their regular minutely anddigitally stepping-up time display job by receiving signal output fromthe decoder 648, as known per se.

On the contrary, when the working voltage at the battery 1 shows anabrupt drop as at 100 in FIG. 1, binary outputs 0 and 1 willalternatively appear at the output of NAND-gate 35, depending upon theinputs from the drive circuit 68.

If occasion may desire, the time display mode can be reversed by slightmodifying the design of the constituent circuit elements in such a waythat with the regular working voltage kept at the battery 1, the displayelements 51B will flicker, while, when the battery voltage should dropabruptly, they illuminate continuously when neglecting the minutesdisplay changes, as may easily occur to those skilled in the art.

In the fourth embodiment shown in FIG. 5, selector circuit 3C comprisesNAND-gates 31C and 32C and an inverter 34C connected with each other andwith the watch movement drive circuit 6C, as shown, which may havesimilar structure and arrangement as those of the foregoing drivecircuit 68..

Amplifier circuit 4C comprises two separate amplifiers 41C and 42C, theoutputs of which are connected to illuminatable or seconds flashingelements 52C and 53C, respectively, mounted on digital display device65C as before. Outputs of NAND-gates 31C and 32C are connected to inputsof these amplifiers 41C and 42C, respectively. These flashing elementsmay preferably be green and red light dissipatable photo-diodes of theirequivalent means.

With such arrangement of the present embodiment, green light flashingcan be performed when the battery voltage is at its regular level, whilered light flashing can be executed after the battery voltage has droppedbeyond a predetermined critical level.

Before describing the fifth embodiment shown in FIG. 8, the principlethereof is briefly illustrated in FIG. 7 which is similar batterycharacteristics shown in FIG. 1. In FIG. 7, the full line curve A issimilar to that denoted A in FIG. 1, denoting that a lightly loadedcondition of a small mercury, silver or the like battery as mostfrequently used one for electronic watches. The dotted line B is thatappearing under heavy loaded condition.

As seen from FIG. 7, when the battery is loaded nearly at the end of itsdurable life, a more predominant voltage drop will be encountered byapplication of a heavier load than the case of a lighter loadapplication. There would be thus such a possibility that a daily, weeklyor monthly application of a heavier load in a sampling manner to abattery normally operating with a lighter load, reliance may be placedupon the aforementioned rather more predominantly appearing voltage dropfor detecting a nearly consumed condition of the battery. The nextfollowing embodiment utilizes such load sampling principle for thedesired purpose.

In FIG. 8, 6D represents schematically an electronic watch drive circuitcomprising an oscillator circuit 61 having a known quartz oscillator.The use of this kind of the high precision timebase can equally beapplied to any of the foregoing embodiments. The circuit 6D furtherconstituents 62; 63a; 63b; 63c; 63d; 63e; 63f; 63g; 64a; 64b; 64c; 64d;64a; 64b; 64c; 64d and 65D connected with each other as shown.

Numeral 62 represents the frequency divider as before, which is of amultistage type, processing the outputs from the oscillator circuit 61into seconds pulse series, as commonly known per'se. From anintermediatestage of the multistage frequency divider, a lead 106 extends so as todeliver therefrom a more frequent l/n-second period pulse series whenassuming that n is a positive integer.

The output from frequency divider 62 is supplied successively toconventional time computers 63a; 63b,- 63c; 63d; 63e, thencesimultaneously to time computers 63g and 63f, all the said computersbeing preferably respective conventional ring counters. In this respect,numerical representations: l/5; 1/12; 1/60; 1/12; 1/2; 1/31 and l/7represent respective frequency-divided ratios. From junction point 107positioned between both blocks 63a and 63b, an output lead 108 extendsfor delivery of S-seconds period pulse series. It will be seen from theforegoing that from the block 63b, 1 minute period pulses series isdelivered to next following block 63c from which 1 hour period pulseseries is delivered. 1n the similar manner, 24-hour period pulse seriesis delivered from junction point 109 positioned between the blocks 63aand 63f and through an output lead 120.

The ratio 1/31 corresponds naturally to the odd month. For even month,the ratio is modified accordingly and manually by supply of a shiftpulse by conventional means coupled with the time-setting stem of thewatch, although not specifically shown on account of its very popularly.

Binary contents of the ring counters 63c; 63d; 63f and 63g are conveyedto conventional decorders 64a,- 64b; 64c and 64d, respectively, theoutputs therefrom are conveyed through respective drivers of theconventional structure to digital illuminatable display elements on thedigital time display device D, as conventionally. As seen from FIG. 8,the device 65D displays, as an example, SUNDAY, 25th day of a month, and12 hours 15 minutes.

The aforesaid l-second period pulse series, shown at a in FIG. 6, willbe conveyed from junction point 109 through output lead 110 extendingtherefrom via a terminal a and further junction 111 to inverter 9bandsimultaneously to one input of NAND-gate 32D included in the selector3D.

The aforesaid 5-second period pulse series, shown at b in FIG. 6, willbe conveyed from junction point 107 through lead 108 including terminalb to inverter 9a, the output thereof being connected to a S-inputterminal of a toggle type flip flop 8a.

The aforementioned 24-hour period pulse series, shown at c in FIG. 6,will be transmitted from junction 109 through output lead 120, includingterminal c, to another Tfinput terminal of the same flip-flop 8a.

The aforementioned l/n-second period pulse series, including terminal d,n being 2 as an example, will be conveyed through lead 106 to an inputof NAND-gate 31D included in the selector 3D. Q-output terminal offlip-flop 8a, the output signal being shown at e in FIG. 6, is connectedthrough inverter and a junction point 119 to T-input terminal of afurther flip-flop 8b. The output of inverter 9b is connected to S-inputterminal of the same flip-flop 8b. Q-output terminal of the latter, theoutput signal being illustrated at f in FIG. 6, is connected through afurther inverter 9d to S-input terminal of RS-flip-flop 8c.

Detector 2D comprises, as before, fixed resistor 23 and variableresistor 22, the input side of the latter being connected with saidjunction 1 19. The input side of inverter 21D is connected to a pointpositioned between the resistors 22 and 23 as before. The output of theinverter 21D is connected through a further inverter 9e to R-inputterminal of the flip-flop 80. Q- output terminal of flip-flop 8c isconnected to another input of NAND-gate 31D. In the similar way,6-outputterminal of flip-flop 8c is connected to another input of NAND-gate 32D.

The positive pole of the voltage source 1, FIG. 8, is connected througha diode 25 to positive terminal of the inverter 21D as hinted at VDD,while the negative terminal thereof is earthed as hinted at VSS. Acrossthis inverter 21D, a capacitor 24 is connected. The outputs ofNAND-gates 31D and 32D are connected to inputs of NOR-gate 32C, theoutput of the latter is connected to a driver 35D, preferably anamplifier, the structure of which is similar to that denoted 35 in theforegoing. The output of the driver 35D is connected to illuminatabledisplay element D which is similar to that denoted 5 in the foregoing.

The operation of the fifth or last embodiment shown in FIG. 8 is asfollows.

As seen especially from at e in FIG. 6, the flip-flop 8a produces asampling signal extending for 2.5 sec once per day. By application ofthis sampling signal, the inverter 90 will act as a conducting switch sothat the VDD-terminal of the voltage source 1 is connected to theseries-connected resistors 22; 23, through the latter, thus, currentflows. Since, in this case, the practical resistance values of theseelements 22, 23 have designed and adjusted to substantially lower levelsthan those which were set in the foregoing embodiments where no suchsampling job is performed, the flow current is stronger than before. Forthis purpose, certain preparatory experiments are executed beforehand,so as to set the passing current positioned on the curve B in FIG. 7, asnearly as possible.

The resistance value at 22 in FIG. 8 has been so adjusted that thevoltage appearing in this case at the intermediate junction 112positioned between the resistors 22 and 23 is slightly higher than thethreshold voltage of the inverter 21D. At the substantial operatingperiod of the battery 1, FIG. 8, signals are implied at the both inletsS and R of RS-flip-flop 8c and the output 6 becomes binary 1, since thesignal pulse implied at the R-input has a longer period than thatappearing the other input S, thereby seconds pulse series being suppliedto the element 5D for flashing the latter as an discriminating visualsymbol to show the electronic watch operates in its regular operatingmode with the battery 1 kept at its regular voltage level along theeffective voltage level as shown by the flat portion of the voltagecurve A shown in FIG. 7.

On the other hand, when the battery 1 is about to be consumed up, thevoltage delivered through junction 112 becomes insufficient to reversethe state of the inverter 21D and thus, input signal will appear only atS- input terminal of the flip-flop 8c, thereby its output Q representingbinary 1. Therefore, the quicker frequency l/n-second period pulseseries will be applied to the element 5D. In this way, the element 5Dflashes at this rather quicker frequency for the display of theabout-consumed condition of the battery 1 as shown at 100' on the dottedline curve B, in advance of the occurrence of the point 100 appearing onthe full line curve A in FIG. 7 by a certain advance period At which maybe days as an example.

The provision of the condenser 24 is for the purpose of keeping theestablished operating voltage level at the inverter 21D than otherwise.In this way, the voltage VDD-VSS as applied to the inverter 21D can bemaintained substantially at a predetermined constant level, if thebattery voltage should drop for a short time period such as 5 seconds asan example.

The provision of the toggle type flip-flop 8b is for the purpose ofmaking the duration terms of the input signal at S-terminal ofRS-flip-flop 8c to be shorter than that of the input signal atR-terminal of the same flipflop 8b.

With the design and arrangement of the present last embodiment so farshown and described, such efficient and advance alarm means may beprovided for the battery-driven electronic watch with least possiblecurrent consumption for such alarm means which consumes an appreciablepower only for each sampling period. Naturally, the above-specifiedsampling period of 5 seconds per day may be modified to any selectedmore economic value.

The embodiments of the invention in which an exclusive property orprivilege is claimed are as follows:

1. In a battery driven electronic watch comprising a watch drivercircuit, a battery for feeding current thereto, and a time-indicatoroperatively connected with said driver circuit, the improvementcomprising a binary detector circuit connected to said battery fordetecting when the source voltage falls below a predetermined level, aselector and drive circuit connected with the output of said detector,and an electrically illuminatable means connected with said selector anddrive circuit, the brightness of said illuminatable means being modifiedfrom its normal state when said source voltage drops below saidpredetermined level in response to the binary logic output of saiddetector, said electrically illuminatable means being mounted on thefront and normally visible surface of said watch.

2. In a battery-driven electronic watch comprising a source battery, anelectronic watch driver circuit fed current therefrom and a digital timedisplay means comprising a plurality of electrically illuminatableelements driven by said circuit, the improvement comprising a detectorfor sensing an abrupt voltage drop at the said battery, and a selectorfor selecting either a first signal series demonstrating the normaloperating voltage kept at said battery or a second signal series of adifferent signal frequency from that of the first signal series anddemonstrating a voltage-dropped condition of said battery, dependingupon the state of said detector and derived from said driver circuit, atleast one of said elements being supplied with either of said bothsignal series depending upon the state of said selector.

3. Electronic watch asset forth in claim 2, further comprising a samplercoupled with said detector and said selector, for impressing forperiodically established short time periods a higher current to saiddetector than that of the normal operating current derived from saidbattery.

4. Electronic watch as set forth in claim 3, wherein said samplercomprises two series-connected toggle type flip-flops and anRS-flip-flop arranged downstream thereof.

5. The electronic watch as set forth in claim 1, wherein saidelectrically illuminatable means comprises an optical display elementwhich is normally in its non-illuminated state but is caused toilluminate when said source voltage drops below said predeterminedlevel.

6. The electronic watch as set forth in claim 5, further comprising asampler coupled with said detector and said selector for impressing forperiodically established short time periods a higher current to saiddetector than that of the normal operating current derived from saidbattery.

1. In a battery driven electronic watch comprising a watch drivercircuit, a battery for feeding current thereto, and a time-indicatoroperatively connected with said driver circuit, the improvementcomprising a binary detector circuit connected to said battery fordetecting when the source voltage falls below a predetermined level, aselector and drive circuit connected with the output of said detector,and an electrically illuminatable means connected with said selector anddrive circuit, the brightness of said illuminatable means being modifiedfrom its normal state when said source voltage drops below saidpredetermined level in response to the binary logic output of saiddetector, said electrically illuminatable means being mounted on thefront and normally visible surface of said watch.
 2. In a battery-drivenelectronic watch comprising a source battery, an electronic watch drivercircuit fed current therefrom and a digital time display meanscomprising a plurality of electrically illuminatable elements driven bysaid circuit, the improvement comprising a detector for sensing anabrupt voltage drop at the said battery, and a selector for selectingeither a first signal series demonstrating the normal operating voltagekept at said battery or a second signal series of a different signalfrequency from that of the first signal series and demonstrating avoltage-dropped condition of said battery, depending upon the state ofsaid detector and derived from said driver circuit, at least one of saidelements being supplied with either of said both signal series dependingupon the state of said selector.
 3. Electronic watch as set forth inclaim 2, further comprising a sampler coupled with said detector andsaid selector, for impressing for periodically established short timeperiods a higher current to said detector than that of the normaloperating current derived from said battery.
 4. Electronic watch as setforth in claim 3, wherein said sampler comprises two series-connectedtoggle type flip-flops and an RS-flip-flop arranged downstream thereof.5. The electronic watch as set forth in claim 1, wherein saidelectrically illuminatable means comprises an optical display elementwhich is normally in its non-illuminated state but is caused toilluminate when said source voltage drops below said predeterminedlevel.
 6. The electronic watch as set forth in claim 5, furthercomprising a sampler coupled with said detector and said selector forimpressing for periodically established short time periods a highercurrent to said detector than that of the normal operating currentderived from said battery.